It is very common in the plastics industry to produce a film of plastic, for example polyethylene, by extruding the plastic material in a tubular shape. In order to achieve the greatest efficiency, the extruders have been made increasingly larger so that the diameter of the extruded tube is quite large. A common means for utilizing these extruded tubular members is to subdivide the large tubular member by flattening the tubular member between pinch rolls, severing the tube longitudinally thereof, and sealing the cut edges to provide a plurality of tubular members. These smaller tubular members can then be cut and sealed transversely to provide a plurality of bags of a reasonably useable size.
The conventional means for carrying out the slitting and sealing operation is through the use of a knife arrangement wherein the knife is heated, and the plastic tubular member is pulled against the sharp edge of the knife, the cut edges then continuing to move in contact with the heated surfaces of the knife member so that the cut edges of the plastic material are melted by the heated knife surfaces, and a bead is rolled at the edge to make a securely sealed edge.
While the prior art slit sealing apparatus has been used reasonably successfully for several years, as the extruders become larger, and the tube is formed faster, the temperature of the knife must be increased in temperature to attempt to form a secure seal and bead in the short length of time the plastic material is riding against the knife. Since polyethylene is basically flamable, it will be understood that an extremely high temperature will tend to burn the material and cause unsightly edges on the product. A lower temperature, with the short dwell times, simply provides for a poor seal which may break under normal use of the final product. One attempt at resolving this problem has been the use of heated air to preheat the plastic material in the area to be slit and sealed so that the subsequent slit sealer can seal more effectively. While such an arrangement may slightly enhance the sealing of the material, such an arrangement will still not assure a good seal at extremely high extruder speeds.